1. Field of the Invention
The invention relates to fueling systems and, more particularly, relates to a fueling system utilizing a diaphragm carburetor to form an air/fuel mixture and to supply the mixture to an engine. The invention additionally relates to an engine fueled with such a system and a method of its use.
2. Discussion of the Related Art
Diaphragm carburetors are widely used to supply fuel to relatively small two-stroke and four-stroke utility engines. A diaphragm carburetor has a diaphragm chamber which opens to main jet and idling jet orifices. Fuel flow through the carburetor is controlled by a regulator located in the diaphragm chamber. The regulator continually opens and closes an inlet needle in response to the vacuum created in the carburetor's venturi. Fuel is supplied to the inlet needle via either a diaphragm pump or by gravity. In the case of the diaphragm pump, suction pulses of the engine are used force fuel through the pump and a series of check valves. The resultant volume of pressurized trapped fuel then flows from the regulator chamber to the fuel jet orifices at a rate that depends on the velocity of the air flow through the venturi which depends on the setting of the throttle valve and the speed of the engine.
Unlike float carburetors, diaphragm carburetors do not have to be vented, and do not rely on the position of a float to maintain a desired volume of fuel in the carburetor. Fuel therefore cannot leak out of the carburetor, even if the carburetor is used on a machine that is subject to severe vibrations and/or that is often operated while inverted or lying on its side. Machines of this type include weed trimmers, chain saws, snow blowers, rammers, and breakers.
A relative disadvantage of diaphragm carburetors is that engines fueled by them can be difficult to start, particularly when the engine has run out of fuel. This is because air can be trapped in the carburetor passage upstream of the diaphragm and in the fuel supply tube leading from the fuel tank to the carburetor. This air must be purged and the diaphragm chamber filled with fuel before the engine can start and run. Depending on the length and diameter of the fuel supply tube, this purging requirement can necessitate 15-20 starting pull cord strokes to purge all of the trapped air. This can be very fatiguing to operators.
Many components have been made and mechanisms implemented for improving the startability of small engines. The most common device used today is a so-called “prime bulb.” A prime bulb is a cap or bulb mounted on or adjacent to the engine and manually activated by an operator to draw fuel into the carburetor and purge air from it. Prime bulbs can be very effective, but they require manual operation apart from the usual starting operation. Operation of a prime bulb may result in the injection of fuel into the throat of the carburetor. Moreover, activation of a prime bulb when the engine is warm, or when the engine fails to start on the first attempt, can flood the engine so that the engine will not start. Moreover, prime bulbs usually are made of rubber or another resilient material that may become brittle with age and with contact with fuel. They therefore have a limited life. This life is further limited by the imposition of shocks and vibrations on the engine during operation of some implements, such as rammers and breakers.
Another technique that is sometimes employed to improve the cold startability of a diaphragm carburetor-equipped engine is a so-called “closed choke,” which is capable of completely or nearly completely closing a choke plate to minimize airflow through the carburetor during a starting operation so as to maximize the richness of the air/fuel mixture. An engine equipped with a closed choke cannot run with the choke fully closed. Instead, the operator must operate the pull cord with the choke closed until he or she detects what is known as a “false hit” in which the engine begins to run but then dies. The operator must then partially or fully open the choke and pull the cord again to start the engine. Closed chokes require even more complex operator interaction than is required for actuation of a prime bulb. They also increase the risk of engine flooding.
The need has therefore arisen to provide a simple, yet reliable mechanism for purging air from a diaphragm carburetor-based fuel supply system in order to facilitate starting of an engine.